A semiconductor wafer (hereinafter referred to as a wafer) is generally a thin disk cut out of a silicon single-crystal ingot, having a diameter of, for example, about 2 to 12 inches. The wafer is provided with many circuits, such as those for LSI's, on its surface and is then cut into chips. The chips are packaged, thus producing semiconductor devices, such as LSI's.
In the above process, the steps of cutting wafers out of the ingot, providing circuits on the wafers, and cutting the wafers into chips and packaging them are often performed in different places. For the transportation of the wafers from a place where one of the above-mentioned steps is carried out to another place, a special container of the wafers which is manufactured for such transportation is generally used.
For example, the wafers are stacked on top of one another and housed in the container. Unfortunately, the stacked wafers are liable to get scratched by their rubbing against each other. It is, accordingly, necessary to interpose wafer protective sheets made of, for example, polyethylene, between the wafers to prevent scratches.
Nowadays, in general, housing wafers in and taking them out of the container are performed by an automatic machine. For example, a pick-up arm having a vacuum suction portion is used for taking wafers out of a container. An optical sensor provided at the vicinity of the vacuum suction portion of the pick-up arm detects a position where wafers are placed and further distinguishes between the wafers and the protective sheets, and the pick-up arm thus transfers the wafers and the protective sheets to their respective places. In that case, since the front and rear surfaces of the wafer are mirror-finished, the wafer protective sheets are adhered to wafers which sometimes causes the automatic machine to be out of order. In order to prevent such a problem, an approach has been proposed in which the wafer protective sheet is provided with projected parts and recessed parts on its surface (refer to, for example, Japanese Patent Application Laid-open No. Hei 9-129719).
In a container in which wafers are housed, pressure is applied to the wafers in their thickness direction by providing cushioning material, etc. at the bottom and top of the inside of the container and covering the container, so that the wafers are prevented from being moved and thus damaged by vibration during transportation. In this instance, if the wafer protective sheets are hard, the projected parts and recessed parts of the protective sheets easily make scratches on the wafers. In order to prevent scratches, a flexible wafer protective sheet made of a nonwoven cloth has been proposed. However, fibers from such wafer protective sheets may negatively affect the wafers due to drop-out of fibers from the wafer protective sheet. It may be considered to arrange a wafer protective sheet to be laminated body and to form its surface by using a soft material. However, this results in high cost, disadvantageously.
Wafers must be free from dust or foreign matter and therefore, wafer protective sheets are hardly reused. In view of disposal, a wafer protective sheet is desired which has a thickness as thin as possible so as to produce few amount of waste.
Accordingly, an object of the present invention is to provide a wafer protective sheet a plurality of which are interposed between wafers stacked on top of one another so as to protect the wafers, which does not adhere to the wafers so as to be easily handled by an automatic machine, which can protect the wafers contained in a container from vibration during transportation, and which is made of a thin material from the viewpoint of disposal.